Acid wash on limestone patio slabs

[layman1]

Hi,

This seems the best place to ask my question. I really hope you can help. Have just had a patio laid by a contractor. Limestone slabs. All going ok until grouting. A residue left on the slabs. Contractor has used a diluted solution of an acid based brick cleaner, apparently on the advice of the slab supplier to remove the residue. The contractor has used brick cleaner with 5 parts water to 1 part cleaner. They have tested this on a few of the slabs. I have to admit it does seem to have improved matters with the residue, but I have said that I want to see the effects after the slabs have dried overnight. I have read horror stories about this type of attempt at cleaning Limestone, not least from this website.

My question is: Will an overnight wait show up any detrimental effects from the acid wash, or would it take longer to show up? If so, how long?

Really hope someone knows the answer to this. I wouldn’t want any more slabs being acid washed if there are longer term repercussions. I did ask the contractor but they are not sure.

Thanks in advance,

A Layman

[lutonlagerlout]

limestone is an alkali
brick acid is a strong acid
the 2 should never mix
its a very bad idea
LLL

[layman1]

limestone is an alkali
brick acid is a strong acid
the 2 should never mix
its a very bad idea
LLL

Thanks for the reply LutonLagerLout. What would you recommend as an alternative to the brick cleaner please? How would you remove the mortar residue (Larsen GPM)? It seems that it is almost impossible to apply it without any residue at all. There must be a way to fix it?

[lutonlagerlout]

not my specialty mate,is the larsen a slurry mix?
Roger may know
cheers LLL

[layman1]

Hi LLL,

It is the streetscape GPM - gunnable pointing mortar - type which was applied directly, but then excess wiped off with a sponge and water. Those marks from the sponge have gone all over the place and it looks a proper mess!

Not too good with these forum things. Not sure how to contact Roger.

Cheers

[Tony McC]

If the acid is going to harm the stone, the effects show up in 24-72 hours.....usually.

However, the harm done to limestone is not usually the stark rust-like markings that can occur with some sandstones, but a degradation of the surface, rendering it chalky, porous and vulnerable. It's quite likely that you would not see any immediate difference, but over the next few months, you may notice the texture is variable and that you're getting areas that are being colonised by algae and lichens.

You have to make a calculation whether the potential risk, which really isn't the end of the world, is preferable to waiting a year or three for natural weathering (which is the same process as the claning acid over a much longer period) for the marks to clear.

[layman1]

Thanks Tony, the guys decided to clean it without an acid based cleaner and instead are opting for a product called Hannafin Oxy-Klenza. The first test area looks promising. Do you know if that product is more gentle and less problematic?

Cheers

[Tony McC]

Oxy-Klenza (FFS! Since when was the inability to spell a viable marketing tool???) is, I believe, like a souped-up version of the Vanish jollop they're always advertising on telly for all sorts of clothing stains.

Personally, I've never tried it, so can't comment from experience, but as a percarbonate it's a) bleeding dangerous stuff, and b) unlikely to make matters any worse.

It would be interesting to hear how you get on. As ever, try a small, discreet area first, and leave it at least 24 hrs before applying it to the wider area.